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Add more Heat - Make the flame larger - Heat up the flue- Make the flue pipe 1/4" per ft and make as short hoz and to flue gas code nor more than 6' Hor. Make as high as can and add flue liner if needed. Brick flue can take 50,000 BTU/ hr to vent put in a flue liner and can take only 250 BTU/hr to heat up to vent. I have seen a brick flue need the hot water heater to stay on 24/7 to vent right and not back draft. It take a lot of heat to keep a brick stack hot.

Ed, just because an appliance backdrafts does not mean you will have any issues with CO as that is created only with the incomplete burning of the fuel. However if all the gasses vent out, an incomplete burn or CO issue is mainly mute from that perspective.

Of course my rule on this is simple - a naturally drafted appliance or one requiring indoor air (yes that includes many power venting units) has no business being in conditioned space and should be removed

Back to studies, unfortunately most deaths or sicknesses that are directly attributed to CO poisoning are missed as the symptoms mimic colds, flus, etc... Not only do we have that issue but CO detectors are only set to go off after they hit what - 70 ppm for a few hours - so many people may never even think that this could be an issue or is the problem

Has anyone done any research on what happens in the real world? Are people getting CO sickness at certain CAZ pressures? We are not picking up any bodies and I am wondering of all of this is necessary. It seems the only CO problems you hear about are blocked chimney's and funky installations.

Often, fixing the venting can do a world of good, at very low cost. Next step, a chimney liner will get a marginal unit to deliver a good solid draft and reliable venting. It's amazing how much better the draft is when the air volume of the flue is reduced to correct proportions.

If the draft is terrible, the chimney liner may not work, and it's costly to experiment. A power vent water heater may be the only good option.

Replacement with an electric water heater is cheap for your program; costly for the occupants. In our region (Wisconsin) electrcity is 2.5 more costly per delivered Btu than NG, about 1.5 times more costly than propane. We see very few oil water heaters.

"Pre-Weatherized" implies "low-income weatherization program" -- in that context, a solar DHW system is neither low-cost nor user-friendly. If you get too far out in front of your skis, the low-income resident will have a really hard time maintaining a renewable system, and will never be able to fix it if it goes south on them.

An aside on solar-thermal, consider that solar-thermal to pay back needs to supply more heating needs and to do that systems likely need to use thermal-fluids, these stay fluid from -30F to 550F so allow the system to store ~550F in the tank, this is about 3-times more therms than water per unit volume that normally is only heated to 150F or so.

This is enough for space-heating by adjusting capacity to needs, then collection added to supply daily capacity.

As soon as a system can supply space-heating it's chewing into bigger bills, infrared is fairly high on cloudy days so gain is there most of the time, yet the system will have a biodiesel backup to heat the fluid when gain falls too low.

Then for distribution consider that fluids to a heat-transfer unit in a room are over 10-times more efficient at moving the heat-cold around than ductwork [so-called ductless typically used with a heat-pump], and the natural system then makes total sense as the payback should be more in line with expectations for adding it.

Not aware of anyone doing this but I will on my next home I own.

My own take is to reduce the need for external energy to where the home is autonomous, collecting high heat from solar-thermal makes more sense as it can supply space-heating and that's the big bills in winter on top of hot-water & laundry.

Home Power Magazine reports that a solar powered hot water heating system which costs $8,000 will not pay back for 27 years, so it's probably best to put in the cheap electric or gas hot water heater. Such is life, when an appliance must pass draft to not kill us with carbon monoxide! [CO is the really bad stuff]. An all electric home is always safest. Once you let the gas gremlin in, watch out!